Breaking a contract between a teacher and school district is a process that requires serious consideration. While many school districts will uphold a teacher's right to quit, or break their contract, at any time, doing so could have serious ramifications on future job searches within the school district, or even in other districts in your area. If you do want out of your teaching contract, the best time to break it is over the summer, before the school year begins so that the school is not left scrambling to replace you.
Read your contract thoroughly. In many states, contracts only last for one school year and must be renewed annually. If you have just received the job offer and have changed your mind, do not sign the contract. The offer will be rescinded with no penalty to you. If you have already signed your contract, it should stipulate a process for breaking it before the term is up.
Speak to your union representative. Teachers' unions exist to protect the rights of teachers when dealing with school districts and administrators. Your representative will be better able to inform you of how to properly break your contract and how doing so will impact your future career path. If you intend to continue teaching, especially within the district, you should seriously consider your reasons for breaking your current contract and talk them over with your union representative to see if there is not another way to resolve the issues.
Draft a resignation letter. Your letter should be professionally written and clearly explain your reasons for breaking your contract. If you have been offered another job, either in another district or as an administrator, say so. If you feel you cannot complete your job requirements as stipulated for any reason, make that clear as well. As a teacher you are responsible for educating children, a job too important to be in the hands of someone who does not want it. Make sure your letter clearly states that you will not be starting your position at the commencement of the school year as originally planned.
Hand-deliver your resignation letter to your district's employment board. Be sure to give at least two weeks before the start of the school year so the district has time to replace you without disrupting students by introducing a new teacher part way through the year.
If your school district offers one-year contracts or yours is up at the end of the school year anyway, consider handing in your resignation at the time new contracts are offered and dating it for the end of the school year. Quitting at the end of a contract term carries no penalties and will not carry the risks to your future career that quitting in the middle of a term will.